Synthesis, Characterization, and Application of Vanadium-Salan Complexes in Oxygen Transfer Reactions

We report the synthesis and characterization of several chiral salen- and salan-type ligands and their vanadium complexes, which are derived from salicylaldehyde or salicylaldehyde derivatives and chiral diamines (1R,2R-diaminocyclohexane, 1S,2S-diaminocyclohexane, and 1S,2S-diphenylethylenediamine). The structures of H(2)sal(R,R-chan)(2+)center dot 2Cl(-)center dot(CH3)(2)CHOH center dot H2O(1c; H(2)sal(R,R-chan) = N,N'-salicyl-R,R-cyclohexanediaminium), Etvan(S,S-chen) (3c; Etvan(S,S-chen) = N,N'-3-ethoxy-salicylidene-S,S-cyclohexanediiminato), and naph(RR-chen) (6c; naph(R,R-chen) = N,N'-naphthylidene-R,R-cyclohexanediiminato) were determined by single-crystal X-ray diffraction. The corresponding vanadium(IV) complexes and several other new complexes involving different salicylaldehyde-type precursors were prepared and characterized in the solid state and in solution by spectroscopic techniques: UV-vis, circular dichroism, electron paramagnetic resonance, and V-51 NMR, which provide information on the coordination geometry. The salan complexes oxidize in organic solvents to V-V species, and this process was also studied using spectroscopic techniques. Single crystals suitable for X-ray diffraction were obtained for [{(VO)-O-V[sal(S,S-dpan)]}(2)-(mu-O)]center dot H2O center dot 2(CH3)(2)CHOH (14c; sal(SS-dpan) = N,N'-salicyl-S,S-diphenylethylenediaminato) and [{(VO)-O-V[t-Busal(RR-chan)]}(2)(mu-O)]center dot 2(CH3)(2)CHOH (15c), both containing an OVV(mu-O)(VO)-O-V moiety (V2O34+ core) with tetradentate ligands and one mu-oxo bridge. Both structures are the first examples of dinuclear vanadium complexes involving the (V2O34+)-O-V core with tetradentate ligands, the configuration of the V2O3 unit being twist-angular. The V-salen and V-salan complexes are tested as catalysts in the oxidation of styrene, cyclohexene, cumene, and methyl phenyl sulfide with H2O2 and t-BuOOH as oxidants. Overall, the V-salan complexes show higher activity and normally better selectivity in alkene oxidation and higher activity and enantioselectivity for sulfoxidation than their parent V-salen complexes, therefore being an advantageous alternative ligand system for oxidation catalysis. The better performance of V-salan complexes probably results from their significantly higher hydrolytic stability. Mechanisms for the alkene oxidation with these newly obtained V-salan compounds are discussed, including the use of DFT for the comparison of several alternative mechanisms for epoxidation.